Due to the deployment of more and more frequency bands for cellular radio-communications applications, the complexity of radio front-end circuitry to be used in such applications increases. Normally, at least one relatively expensive external (or “off-chip”) filter, usually a SAW (Surface Acoustic Wave) filter and/or duplexer, has to be used for each frequency band to be received with the cellular radio. Therefore the size and cost of the external front-end components increase when introducing new frequency bands. Striving towards more flexible front-end solutions requires new circuit solutions that can handle strong interferers and prevent harmonic down-conversion without sacrificing any other performance.
It is thus desirable to provide radio front-end circuitry that eliminates the need for at least some of the off-chip filters, e.g. relatively expensive SAW filters and/or duplexers, that are normally used in present radio communications circuitry, or that at least relaxes the requirements on such off-chip filters, which in turn facilitates a reduction of manufacturing cost.
To handle the strong out-of-band interference in cell phones, or other similar communication devices, without using sharp radio frequency (RF) filters, such as SAW filters and/or duplexers, a relatively high linearity is normally needed. Otherwise the unfiltered amplified interference might saturate low-noise amplifiers (LNAs) or down-conversion mixers. To reduce the interference, it has been proposed (e.g. in US 2005/0239430 A1) to use so called harmonic rejection in down-conversion circuitry to suppress interference at harmonics of a local oscillator (LO) signal, which is particularly useful in a radio receiver without sharp RF filters, since interferers at harmonics of the LO signal will be down-converted to baseband and, unless suppressed in some way, detrimentally interfere with the (desired) baseband signal.
WO 2008/139390 A1 discloses a mixer circuit, wherein an input signal is switched in accordance with a first local oscillator signal and in accordance with at least one second local oscillator signal having a smaller duty cycle than said first local oscillator signal, or having a respective predetermined phase shift with respect to said first local oscillator signal. Output signals obtained by the switching in accordance with the first and at least one second local oscillator signals are summed and the polarity of one of said first local oscillator signal and said at least one second local oscillator signal is switched in response to a control input, to thereby switch between a harmonic-rejection mode and a sub-harmonic mixing mode.